Protective device



G. CAMILLI PROTECTIVE DEVICE Filed Jan. 3, 1940 July 8, 1941.

Inventor: Gucgi'ielmo Camilli His Attorney.

Patented July 8,' 1941 UNITED STATES PATENT OFFICE General Electric CoNew York mpany, a corporation of Application January 3, 1940, Serial No.312,195 1 Claims.

My invention relates to protective devices and pertains particularly toapparatus for limiting over-voltage.

It is an object of my invention to provide arrangements for protectingcurrent transformers, preventing excessive voltages in the secondarywindings, and preventing destruction of protective shunts which may beconnected across the secondary windings to limit the voltage.

Other and further objects and advantages will become apparent as thedescription proceeds.

In carrying out my invention in its preferred form I provide aprotective resistor composed of material having a hyperbolicresistance-current characteristic for connection across the secondarywinding of a current transformer. I also provide a thermal current relayhaving an inverse current-time characteristic with an operating windingconnected in series with the protective resistor. The relay includesnormally open contacts connected across the secondary winding of thetransformer so that as the current builds up in the protective resistoras a result of an inadvertent disconnection of the normal burden fromthe current transformer, the current through the relay winding willcause the closingof its contacts to prevent the continuance of currentflow through the protective resistor, which might otherwise be heated todestruction unless a resistor of uneconomical and inefficient size wereemployed.

The invention will be understood more readily from the followingdetailed description when considered in connection with the accompanyingdrawing and those features of the invention which are believed to benovel and patentable will be pointed out'in the claims appended hereto.In the drawing, Figure 1 is a side elevation of apparatus forming oneembodiment of my invention, a portion of the apparatus being broken awayin order to expose the interior. Figure 2 is a vertical section of theapparatus of Figure 1 cut by the plane 22 and Figure 3 is a schematicdiagram showing the electrical connections of the apparatus and aperspective representation of the relay. .Like reference characters areutilized throughout the drawing to designate like parts.-

Current transformers are employed both to insulate current measuringinstruments from highvoltage alternating-current circuits and also topermit the measurement of high currents with ammeters designed forrelatively small currents, such as the standard full scale deflection offive amperes, for example. Current transformers are designed to havetheir burden, in the form of ammeters or current coils of various typesof instruments, meters and relays, continuously connected to thesecondary winding. If the burden should inadvertently be disconnected,dangerously high voltages are produced in the secondary windings. Thesevoltages must be guarded a ainst both to avoid the hazard to human lifeand also to prevent the breakdown of insulation of the transformer.

In the arrangement illustrated, a current transformer H having asecondary winding 12 is provided with a protective resistor l3 connectedacross the secondary winding l2 to prevent the voltage from rising toexcessive values. It will be understood that the normal burden of thecurrent transformer, such as an ammeter I4, is connected to outputterminals l5 which are also connected across the secondary winding H. Toprevent the protective resistor l3 from being destroyed by excessivecurrent when the normal burden H is inadvertently not connected, athermal current relay I6 is provided.

The protective resistor I3 is what is commonly called a non-ohmicresistor having a non-linear voltage current characteristic. It iscomposed of some special form of resistance material which has theunique property of decreasing in resistance with increase in current, sothat as the voltage across it rises slightly, causing an increase incurrent, the decrease in resistance prevents the voltage from increasingsubstantially regardless of the extent to which the magnitude of thecurrent may rise. For this purpose a resistor may be employed consistingof a material having what may be called a hyperbolic resistance-currentcharacteristic. That is to say the material satisfies the equationRI"=C. This equation, expressed in terms of voltage, may be written 1I=K.E =K.E

Resistance, current and voltage are represented in the equation by thesymbols R, I and E, respectively, and a, C, K and b are constantsdetermined by the material. Certain materials are available in which theexponent a may be made very nearly unity, thus making the exponent brelatively great. A suitable material is one of the type disclosed inLetters Patent of the United States 1,822,742 to Karl B. McEachron,assigned to the General Electric Company, ranted September 8, 1931, fora discharge device and re.

sistance material. Such a material may readily be produced having anexponent :1 approximately .9 making the exponent b approximately 10 sothat the current through the resistor may rise very rapidly with aslight rise in voltage. .As is known to those skilled in the art, thischaracteristic of the resistor is not a temperature eflect but isindependent of temperature. The material employed is preferably amixture oi silicon carbide and carbon with a suitable binder or ofsilicon carbide mixed with other suitable materials such as tungsten andmolybdenum and the like, but it will be understood that any suitablematerial having the required property of decreasing in resistance withan increase of applied voltage may be employed.

In the arrangement illustrated in the drawing the protective resistor l3consists of a pair of disks l1 and I8 composed of the non-ohmicresistance material and having the surfaces sprayed with a metalliccoating to facilitate making good electrical and thermal contact. Inorder to provide heat-radiating fins the two resistor disks are placedbetween three disks I9, 20 and 2| composed of a material which is a goodconductor of electricity and heat, such as copper, for example. Thedisks or tins I9, 20 and 2i extend well beyond the resistance disks iiand i8 in order to provide ample radiation and convection surfaces forthe dissipation oi. heat, and the outer heat radiating disks l8 and Elare preferably dished in order to increase their efhciency in relationto the center disk 20. The disks l1 and it are arranged in parallel toform a resistor unit, electrical connections being made by means of aconductor 22 to the center disk 20 and a conductor 23 to the outer disksl9 and 21.

For mounting and supporting the resistor unit 13 a suitable frame workconsisting of flanged plates 24, insulating spacing washers 25 and aclam-ping bolt 26 are provided. It will be understood that the heatradiating disks IS, 20 and 2| and the resistance disks l1 and N areprovided with suitable openings to admit the bolt 28. Preferably aninsulating sleeve 21 is provided around the bolt 26.

The thermal current relay l includes a helical bimetallicheat-responsive operating member 28, a current-conducting heat-producingoperating winding 29 surrounding the bimetallic member 28, and a pair ofcontacts 30 and 2|, the first of which is movable in response totwisting of the bimetallic helix 28. In order to give snap action to themovable contact 30 there is provided a bowed column spring 32 supportedin a stii'ily resilient U-member 33 supported by the stationary portionof the apparatus. The helix 28 is rigidly supported at one end 34 andthe movable end 35 carries a forked crank 38 engaging the column spring32 near one end thereof. The movable contact 30 is carried by aresilient strip 31 supported at one end 38 and formed as a fork 39 atthe other end, engaging a center portion of the column spring 32.

The relay i6 is mounted in a suitable casing III of insulating materialwhich may consist of a molded synthetic resin, for example. The relaycasing I0 is secured against an insulating sheet 4| which in turn ismounted on one 01' the frame plates 24. To permit making connections tothe disks l1 and I8 from the back of the plate 4|, an opening 42 ispreferably cut in one o! the frame plates 24 and the insulating sheet. His mounted against the opening 42.

ing heater winding 2!, and a conductor 44. It

will be understood that the primary winding of the transformer H isconnected in series with a circuit in which the current is to bemeasured.

If the ammeter H should be inadvertently disconnected, removal of theburden would cause the voltage of the secondary winding II to rise.Although this rise in voltage is limited by the protective resistor II,it is sufllcient by reason of the characteristics or the resistor II tocause an increase in current through the relay heater winding 29, whichthereupon moves .the switch blade 21 and closes the contacts II and I I.which short circuit the winding [2 through the conductor 45, contacts IIand 30, the blade 31 and the conductor 44, thereby terminating thecurrent flow in the resistor disks I1 and IO and preventing theirdestruction. As previously explained the material of which the resistordisks l1 and i8 is composed has the property 0! diminishing inresistance very rapidly with an increase in current so that a veryslight rise in voltage produces a very great rise in current which, oicourse, places a burden on the transformer H and prevents rise involtage to excessive values. Owing to this property of tailing from arelatively high resistance to a relatively low one with slight changesin voltage, I may use a resistor unit II, which under normal conditions,has such a high resistance as to have no observable eflect on thecurrent transformer l2, thus making it permissible to attach theprotector system described in this application to existing currenttransformers in the field without any recalibration.

By utilizing resistance disks approximately 3 inches in diameter andinch in thickness, I may limit the secondary voltage 01' the transformerto approximately 180 volts. So long as the secondary voltage remainsbelow volts the current through the disks l1 and I8 remainsimperceptible. But when the voltage rises above 80 volts the currentdrawn exceeds 1.5 amperes. The thermal relay I0 is so set that currentpassing through the resistor unit II for prolonged periods of time at 80volts causes the closing of the contacts 30 and 2|. The relay I8 is soarranged as to have an inverse current-time characteristic. In case oivery heavy transient current which might destroy the resistors l1 and IIin a relatively short period of time, the relay ll acts very quicklythus closing the contacts II and SI almost instantaneously. On the otherhand, in the case of small over-voltages gradually applied, the contactsII and II do not close until such a period oi time has elapsed that theheating of the resistors I l and II might become excessive. The closingof the contacts II and II, of course, stops the flow of current throughthe heater wire 29 and after approximately two minutes the contacts 3|and 1| reopen as a result 01' the cooling of the winding 29. However, itthe over-voltage is still present in the secondary winding I2, thecontacts II and II begin to reclcse, the characteristics of theresistors l1 and I8 preventing the voltage from exceeding volts in anyevent. Fast snap action of the movable contacts 30 is obtained by reasonof the fact that the bowed column spring 32 must be flexed in either onedirection or the other, a relatively small motion of the actuating fork36 at the end of the spring 32 being suflicient to snap it from theposition shown in which spring 32 is concave forward to the position inwhich it is convex forward. Due to the fork 39 being at the center ofthe spring 32, relatively great motion of the blade 31, sufficient toproduce positive opening and closing of the contacts, is provided.

In accordance with the provisions of the patent statutes, I havedescribed the principle of operation of my invention together with theapparatus which I now consider to represent the best embodiment thereofbut I desire to have it understood that the apparatus shown is onlyillustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An over-voltage protector for current transformers comprising aprotective resistor having a hyperbolic resistance-currentcharacteristic, heat radiating fins of high thermal conductivity inintimate surface contact with the resistor and extending therebeyond,and a thermal relay withan inverse current-time characteristic, having apair of normally open contacts adapted to be connected across thesecondary winding of a current transformer to be protected and having anoperating winding normally connected in series with the resistor to thesaid contacts, whereby a transformer winding connected to said contactsnormally has a series circuit connection through said resistor and saidoperating winding and closure of the contacts short circuits such atransformer winding and shunts out the said resistor and relay operatingwinding.

2. An over-voltage protector for current transformers comprising aprotective resistor comprising non-ohmic resistance material, and arelay with an inverse current-time characteristic, having a pair ofnormally open contacts adapted to be connected across the secondarywinding of a current transformer to be protected and having an operatingwinding normally connected in series with the resistor to the saidcontacts, whereby a transformer winding connected to said contactsnormally has a series circuit connection through said resistor and saidoperating winding and closure of the contacts short circuits such atransformer winding and shunts out the said resistor and relay operatingwinding.

3. A voltage limiter for an electrical circuit comprising a voltagelimiting resistor having an hyperbolic resistance-currentcharacteristic, and a relay for protecting the resistor againstprolonged excessive current, said relay having a pair of normally opencontacts adapted to be connected across the electrical circuit in whichvoltage is to be limited, and an operating winding normally connected inseries with the resistor across the said electrical circuit, wherebysaid electrical circuit normally has a series connection through saidresistor and said relay operating winding and closure of the contactsshort circuits said electrical circuit and shunts out the said resistorand relay operating windin said operating winding being dimensioned toclose the said contacts when the current reaches a value which woulddamage the resistor during the period of current duration, whereby theresistor protects the electrical circuit against overvoitage and therelay in turn protects the resistor against over-heating.

4. A current transformer having a secondary winding, an open circuitprotective resistor therefor comprising non-ohmic resistance materialwhich falls off in resistance in response to increase in voltageindependent of temperature effects, and a relay with an inversecurrent-time characteristic having a pair of normally open contactsconnectedto the respective ends of the secondary winding of the currenttransformer and having an operating winding normally connected in serieswith the resistor to the said contact-s, whereby the said transformerwinding normally has a series circuit connection through said resistorand said relay operating winding and closure of the contacts shortcircuits the said transformer winding and shunts out the said resistorand relay operating winding.

5. The combination of a current transformer having a secondary winding,an open circuit protective resistor therefor having an hyperbolicresistance-current characteristic, and a relay having a pair of normallyopen contacts connected to the ends of the secondary winding of thecurrent transformer and having an operating winding normally connectedin series with the resistor to the said cont-acts, whereby the saidtransformer winding normally'has a. series circuit connection throughsaid resistor and said relay operating winding and closure of thecontacts short circuits the said transformer winding and shunts out thesaid resistor and relay operating winding, said current transformerbeing designed for normally having a burden connected to the secondarywinding and having the characteristic of producing unsafe secondaryvoltages when the burden is accidentally disconnected, said protectiveresistor and said relay being so dimensioned that, when the secondaryvoltage rises sufficiently to cause a burden current to flow through theprotective resistor sufiicient in value to limit the secondary voltageto a safe value, the said burden current is of suflicient magnitude toclose the contacts of the relay, whereby the protective resistor isitself protected against over' heating.

GUGIIELMO CAIVHLLI.

